1. Field
One embodiment of the invention relates to an optical disk drive which records information in an optical disk such as a DVD or a CD or which reproduces the information recorded in the optical disk, more particularly to a light receiving element circuit to be applied to an optical disk drive.
2. Description of the Related Art
In a case where information recorded in, for example, an optical disk is reproduced by a disk drive built in a personal computer or an optical disk drive such as a DVD recorder, a recording surface of the optical disk is irradiated with laser light from an optical pickup which moves in a radial direction of the optical disk. The laser light reflected on the recording surface is received by a light receiving element in a light receiving element circuit (in general, a semiconductor integrated circuit) disposed in an optical pickup to generate a photo detection signal. On the basis of this photo detection signal, laser light focusing, tracking and reproduction of information are performed.
In the light receiving element circuit, the reflected light from the optical disk recording surface is received by, for example, four divided light receiving elements. Each light receiving element photoelectrically converts the received reflected light to generate a current in accordance with intensity of the reflected light. This current is amplified by a current amplifier, further amplified by a voltage amplifier, and then supplied as the photo detection signal. In general, this photo detection signal is supplied to a signal processing unit fixed to the optical disk drive via a flexible cable such as a flat cable.
The signal processing unit generates servo signals such as a focusing control signal and a tracking control signal as well as an information reproduction signal based on the photo detection signal supplied from the optical pickup (light receiving element circuit). The servo signal generated by the signal processing unit is supplied to the optical pickup via the flat cable, and used for laser light focusing control and tracking control.
The flat cable for use in the optical pickup requires a large number of core wires such as a signal transmission line of the photo detection signal or the servo signal described above and a power supply line. In Jpn. Pat. Appln. KOKAI Publication No. 2004-273680 (FIG. 2), there is disclosed a technology to reduce the number of the core wires of the flat cable for use in the optical pickup.
In recent years, as to the optical disk drive, types of optical disk drives have increased in which a plurality of tracking systems are adopted in order to reproduce various types of optical disks such as a DVD-ROM and a DVD-RAM. In order to perform the focusing and the tracking correctly at a high speed with respect to an optical disk such as an HD DVD in which the recording is possible with a higher density than before, the light receiving element circuit is generalized in which the laser light is divided using a hologram (optical diffraction element), and the reflected light is received by a light receiving unit having more light receiving elements than before.
In a case where the reflected light is received by the light receiving unit adopting a plurality of tracking systems and having more light receiving elements than before in this manner, a circuit constitution of the light receiving element circuit becomes complicated, power consumption of the circuit increases, and the flat cable for signal transmission further requires more core wires. In recent years, the optical disk drive has been used in a video camera or a mobile product such as a car navigation system, and there is a strong demand for reduction of the power consumption and space saving.
Moreover, in recent years, with high densification of the disk, the optical disk drive requires a reduction in a read/write time of the information with respect to the disk, and an increase of a response speed of the drive. Therefore, in general, a constitution is adopted in which a high frequency (HF) signal output from the light receiving element circuit has a form of a positive-phase/negative-phase differential output signal. According to such constitution, the number of the core wires of the flat cable further increases.
In the form of the differential output signal, a positive-phase signal and a negative-phase signal are simultaneously output which have a reverse-phase relation to each other. In the signal processing unit which receives the signals, when a difference between the positive-phase signal and the negative-phase signal is generated, a signal amplitude is doubled to enlarge a dynamic range of the signal. Furthermore, there are removed in-phase noise components included in the positive-phase signal and the negative-phase signal. Therefore, transmission of the signal in the form of the differential output signal has an advantage that the transmission is resistant to noise.
On the other hand, with the increase of the speed, the power consumption of the disk drive increases. Therefore, there is a large problem that a heat rise in the disk drive increases, and yield of the disk drive and margins of performance decrease.
In general, the light receiving element circuit is configured to output an original signal of the servo signal generated in a signal processing circuit in addition to an HF signal. There are arranged several tens of light receiving elements at the most. This increases amplification elements, and also increases a scale of the circuit. This results in an increase of the power consumption of the light receiving element circuit itself.
In Jpn. Pat. Appln. KOKAI Publication No. 2002-198748, there is disclosed the light receiving element circuit of such differential signal output form. In this document, the light receiving element circuit is described in which a reference voltage of a positive-phase-signal-side amplifier and a reference voltage of a negative-phase-signal-side amplifier are generated from a reference voltage supplied from the outside of the light receiving element circuit, and the signal is output in a differential form.
In the light receiving element circuit of Jpn. Pat. Appln. KOKAI Publication No. 2002-198748 described above, as the reference voltages in a differential signal output form amplification circuit disposed for each light receiving element, three reference voltages are used which include: the reference voltage input from the outside; the positive-phase reference voltage generated in the circuit; and the reverse-phase reference voltage generated in the circuit. A voltage which is higher than the reference voltage input from the outside by a predetermined level is used as the reference voltage for the positive-phase signal, and a voltage which is lower than the reference voltage input from the outside by a predetermined level is used as the reference voltage for the negative-phase signal.
In such light receiving element circuit, the circuit constitution becomes complicated, and the dynamic range of the signal is easily broadly taken, but a consumed current increases as much as the range.
In recent years, several tens of light receiving elements are used in the light receiving element circuit, and the above-described amplification circuit of the differential signal output form is required for each light receiving element. Therefore, there is a strong demand for simplification of the signal amplification circuit and reduction of power consumption.